1. Technical Field
The invention is related to a high density plasma CVD reactor having inductive and capacitive coupling modes which can be selected individually or in combination for chamber cleaning operations.
2. Background Art
Plasma reactors used for semiconductor processes such as chemical vapor deposition (CVD), etching, reactive ion etching and so forth typically employ either inductive coupling or capacitive coupling to strike and maintain a plasma. Typically, an etch reactor employs capacitive coupling because the high ion energies characteristic of capacitively coupled plasmas are suitable for removing films such as, for example, silicon dioxide. The high ion energies arise from the high electric fields required to strike a capacitively coupled plasma. For example, as disclosed in European Patent Application publication No. 0 552 491 A1 by Collins et al. entitled "Plasma Etch Process" published Jul. 28, 1993, a capacitively coupled reactor has parallel electrodes, including a grounded electrode in the ceiling and an RF biased electrode in the wafer pedestal.
However, the plasma density of a capacitively coupled plasma is not sufficient for performing simultaneous chemical vapor deposition (CVD) and in-situ sputtering at low pressures. Accordingly, a CVD plasma reactor of the type disclosed in U.S. patent application Ser. No. 08/113,776 entitled "High Density Plasma CVD and Etching Reactor" filed Aug. 27, 1993 by Fairbairn and Nowak employs inductive coupling to strike and maintain the plasma. In the Fairbairn and Nowak application, a domed-shaped helical coil in a dome-shaped chamber ceiling performs the inductive coupling to the plasma at a low chamber pressure (below 100 milliTorr), the dome shape providing a more uniform ion density distribution across the semiconductor wafer. Such an inductively coupled plasma is characterized by a high ion density and is therefore suitable for low pressure CVD plasma processing at high processing (e.g., deposition) rates.
The main differences between capacitively coupled plasmas and inductively coupled plasmas are the following: An inductively coupled plasma has a plasma density which is up to two orders of magnitude higher, thereby providing higher processing rates. A capacitively coupled plasma has a higher electrode sheet voltage, higher self-biasing and higher plasma impedance, and therefore provides higher sputtering rates and greater risk of damage to the wafer. An inductively coupled plasma exhibits lower ion energy distribution, providing a lower risk of damage to the wafer.
A requirement of all plasma reactors, and especially plasma CVD reactors, is that they must be cleaned periodically to remove deposits and residue. For example, in a CVD reactor, during deposition of silicon dioxide onto a semiconductor wafer, silicon dioxide is deposited onto the chamber walls and ceiling as well as other parts of the reactor chamber, changing the characteristics of the chamber and giving rise to particulate contamination. Therefore, the reactor chamber must be cleaned periodically to remove all such deposits and residue therefrom. Removal of silicon dioxide deposits is accomplished by introducing a fluoride-containing etchant gas such as NF.sub.3 (for example) into the chamber and striking a plasma to perform a chamber cleaning operation.
One limitation of inductively coupled plasma reactors is that they require more time to perform a chamber cleaning operation due to their larger volumes, low operational pressure, non-conductive ceilings and lack of bias of the chamber wall surfaces. Certain residues, such as silicon dioxide, typically have relatively low etch rates at low ion energies, and therefore can be difficult to remove by inductively coupled plasmas.
One problem with such a chamber cleaning operation is that the various surfaces of the reactor chamber interior are not cleaned (etched) at the same rate, due to differences in location in the chamber and differences in materials. The plasma density and ion energies are not the same at all locations in the chamber, so that differences in cleaning (etch) rates are quite typical. Moreover, during wafer processing (such as CVD processing), deposits build up more thickly on certain chamber surfaces than on others, so that the deposit or residue thickness is non-uniform throughout the chamber interior. For example, in the etch reactor of the Collins et al. European application referenced above, the etch-process residue is much thicker in the center of the ceiling than at the edge of the ceiling, due to plasma density non-uniformity.
As a result, some metal surfaces are thoroughly cleansed of residue or deposits before others, and the cleaned metal surfaces (e.g., aluminum) are etched while the remaining contaminants are removed from the other still-unclean surfaces. For example, in the case of the flat ceiling electrode of the Collins et al. European application, the edge of the ceiling electrode quickly becomes clean while the center portion still has a thick residue coating remaining thereon. The exposed metal surface of the electrode edge shunts RF energy away from the still-unclean center portion, thereby preventing any further cleaning of the center portion. If a fluoride gas such as NF.sub.3 is employed to clean the chamber, the metal surfaces react with the etchant gas during the cleaning operation to form various aluminum fluorides, which redeposit onto various chamber surfaces, including the ones that are still unclean with other residues or deposits. Such aluminum fluorides etch very slowly or are virtually impervious to being etched (particularly in an inductively coupled plasma), thus masking the prior deposits (e.g., silicon dioxide) underneath. This deposition of aluminum fluorides during the cleaning operation prevents the removal of the remaining residues (e.g., silicon dioxide), so that the reactor chamber cannot be thoroughly cleaned, even by extending the duration of the chamber cleaning operation.
A related problem is that some chamber dielectric surfaces (for example, quartz surfaces) are consumed rapidly during a chamber cleaning operation, particularly in an inductively coupled plasma, and must therefore be frequently replaced at great cost, a significant disadvantage.
A problem also exists in regards to the standard radial gas distribution systems employed at the periphery of the base/bottom of the source region of the chamber, such as the system designated G1 in the aforementioned European Patent Application publication No. 0 552 491 A1. These systems typically have a gas distribution ring with inwardly facing radial holes. When these types of systems are used to emit gas, (e.g. silane), during a CVD process to form an oxide layer over the surface of the wafer (e.g. SiO.sub.2), the emitted gas tends to diffuse equally in all directions, not just toward the wafer. In the case where silane is being used as the emitted gas, the silane and oxygen from the plasma react together spontaneously. Since the chamber walls are closer to the gas outlet holes than most of the wafer (particularly for larger diameter wafers), deposition of SiO.sub.2 over the interior surfaces of the vacuum chamber is greater than that on the wafer. This means that the reactor must be periodically removed from productive activity and the SiO.sub.2 coating removed from its interior surfaces, a significant disadvantage.
Thus, there is a need for a reactor which deposits less CVD residue (e.g., SiO.sub.2) on the interior chamber surfaces, and which therefore, requires less frequent cleaning.